1. Field of the Invention
The present invention relates to an electronic circuit for driving a plurality of serially-connected elements and more particularly, to a series drive circuit for driving a plurality of serially-connected electronic elements, such as electrochromic mirrors or electrochromic windows, and maintaining a relatively a accurately controlled voltage across each electronic element while minimizing power dissipation.
2. Description of the Prior Art
Variable electrochromic mirror and window systems (hereinafter "electrochromic elements") are generally known in the art. Such systems normally include a plurality of electrochromic elements. For example, in an automotive application, electrochromic elements are known to be used for both the rearview mirror and one or more of the sideview mirrors or window applications for sun load control. It is known that the reflectance of an electrochromic element is a function of the voltage applied to the electrochromic element, for example, as generally described in U.S. Pat. No. 4,902,108 assigned to the same assignee as the assignee of the present invention and hereby incorporated by reference. Because of this characteristic, such electrochromic elements are known to be used to automatically control glare from external light sources in various automotive and other applications. In such applications, a plurality of electrochromic elements are controlled. More particularly, in some known systems the electrochromic elements are known to be generally connected in parallel and sourced from a common source of electric power. The drive circuit may be powered from a 12 V, 8 V or 5 V. In an automotive application, the 12 volt vehicle battery is used as the electrical power source for the electrochromic elements. The actual element voltages are less than 1.5 V. Since the actual element voltages are low compared to the supply voltage, multiple elements may be connected in series to provide multiple element drive with current consumption equivalent to a single element to reduce power dissipation in the drive circuitry.
In order to prevent damage to such electrochromic elements and actually control reflectance of the various elements, the voltage across each element must be controlled. Unfortunately, it is relatively difficult to dissipate the required power in a small area, such as inside an automatic mirror, with multiple electrochromic elements in a parallel configuration, as discussed above. In particular, it is known that the effective resistance of the electrochromic elements varies as a function of part-to-part variations in the electrochromic elements. It is also known that the resistance of the electrochromic elements may vary as a function of temperature. In applications where the electrochromic elements are used both inside and outside the vehicle, the temperature difference between the inside and outside electrochromic elements can be relatively significant. In applications where the electrochromic elements are connected in parallel, the electrical current, and thus, power consumption of the various electrochromic elements can vary significantly.
Previous attempts have been made to resolve the problem of power dissipation discussed above. In one known previous attempt, the various electrochromic elements were connected in series and driven from a common source of electrical power. Unfortunately, in that known attempt, it was relatively difficult to control the voltage across each of the electrochromic elements in order to prevent damage and accurately control reflectance.